Programmable Humidity Actuators Enabled by Precise Interfacial Control of COF Nanostructures on Nanocellulose

Designing humidity-driven actuators with rapid, programmable motion remains challenging due to limited responsiveness and poor mechanical integrity in conventional materials. Here, we demonstrate a strategy that transforms one-dimensional (1D) cellulose nanofibers (CNFs) into mechanically robust two-dimensional films through premodification, which simultaneously directs the controlled growth of covalent organic frameworks (COFs) to produce tunable, moisture-responsive surfaces. Fourier transform infrared spectroscopy , ζ-potential, scanning electron microscopy, X-ray diffraction, and X-ray photoemission spectroscopy analyses confirm uniform modification and distinct COF nanostructures: covalent anchoring on reactive CNFs generates vertically aligned nanoflakes, whereas electrostatic catalysis on charged CNFs yields densely packed nanorods. The resulting COF@CNF films exhibit enhanced tensile strength (∼230 MPa vs 150 MPa for pristine CNFs), folding endurance (up to 1.6 × 10⁴ cycles vs under 4 × 10³ cycles on COF@polypropylene), and hydrophobicity (∼110°) while maintaining flexibility. Asymmetric COF deposition enables programmable humidity-driven bending and complex motions, including self-oscillation and S-shaped deformation. This work illustrates how the precise nanoscale design of 1D building blocks drives emergent bulk properties, offering a versatile platform for high-performance, programmable actuators.